The present invention relates to a pressure sensor, or more specifically to an optical pressure sensor (pressure sensor element) which measures pressure applied from the exterior by detecting the intensity fluctuation of the light transmitted through a polymer optical conductor.
The resistance strain gauge and the semiconductor pressure sensor are conventional and well known in the art as pressure sensors. These sensors convert the detected pressure into an electrical signal. Therefore, if the detector is located in an environment near a copying machine or a like high potential source, in a vehicle with extreme temperature variation or in water, the electromagnetic atmosphere can cause noise in the signal, or the ionic atmosphere can lead to corrosion. Besides, if the sensor is installed in a combustible atmosphere, electrical spark may cause fir or an explosion.
In view of the foregoing, there is proposed an optical pressure sensing system as an improved pressure detection system with a good resistance to the environment, free from electromagnetic noise influence and explosion or fire possibility, while capable of detecting signals from the remote sensor element through an optical fiber.
There are a number of different types of optical pressure sensors: a switch type sensor which mechanically blocks the light path according to the pressure; a sensor which uses the photoelastic effect in detecting the light intensity fluctuation due to the pressure variation; and a sensor which uses the acoustic optical effect in detecting vibration. For any of these types, if the sensor is to detect the light intensity using the plane of polarization of the light, it involves not only a sensing substance but also a polarizer, an analyzer, a polarized beam splitter, a .lambda./4 wavelength plate and a rod lens for connection between the sensor element and the optical fiber, and it is difficult to assembly these optical components into a compact sensor. In addition, the necessity of fixing the optical components with the sensing substance makes batch-processing of the sensor element difficult. Besides, considerable expertise is required in the above fixing operation. As a result, the sensor element production requires an extremely high cost.
There is another type of sensor that measures pressure fluctuation by detecting light intensity change due to interference. An example of this type is a Michelson interferometer of a Mach-Zehnder interferometer in which an inorganic photo conductor made of LiNbO.sub.3 is used as a sensor element. Using the optical conductor, this type of sensor involves a reduced number of optical components, and the sensor element can be batch-processed. On the other hand, however, this sensor has the following disadvantage: since the sensor detects pressure based on the interference, it must use monochromatic light as the detection signal. Accordingly, it is necessary to use single-mode optical fiber for transmitting detection signals, though low loss connection between the sensor element and the single-mode optical fiber is quite difficult. Another possible pressure sensor involves a sensor element having a hollow structure composed of mica connected to the single-mode optical fiber end, so that the wall-to-wall distance variation of the hollow structure due to the pressure fluctuation is detected the form of the reflected or transmitted light intensity change due to Fabry-Perot interference. This type of pressure sensor has an advantage in that it can be made into a compact or probe construction. However, it is disadvantageous in that the sensor element has no compatibility and reproducibility in production: a hollow sensor element with wall-to-wall distance of wavelength magnitude is difficult to produce with good reproducibility.